1. Field of the Invention
This invention relates to structures employing semiconductor powders and thin catalyst films which can photosensitize chemical reduction-oxidation (redox) reactions and simultaneously provide an electrical output which is a measure of one of the chemical species involved in the reaction.
2. Prior Art
FIG. 1 shows a schematic diagram of a prior art, conventional photoelectrochemical (PEC) cell 10 in which a semiconductor 11 of macroscopic dimensions is connected by an external circuit 12 to a counter electrode 13. Both semiconductor 11 and counter electrode 13 are immersed in an electrolyte solution 14, advantageously aqueous, which contains chemical species (redox species identified as D- and A+) which are subject to either oxidation or reduction reactions that occur with the transfer of electronic charge at the surfaces of semiconductor 11 and counter electrode 13.
Light of energy greather than the energy of the semiconductor electron bandgap is made to illuminate the semiconductor. As a result, pairs of free electrons 15 and holes 16 are generated near the illuminated surface. Under appropriate conditions for the energy level position of the redox levels in the electrolyte relative to the conduction and valence band energy positions in the semiconductor, the electrons and holes can transfer to the redox species thereby effecting the redox reactions. The various processes and conditions involved are discussed by H. Gerischer in Physical Chemistry--An Advanced Treatise, H. Eyring, D. Henderson, W. Jost. Eds. (Academic Press, New York, 1970), pp. 463-542.
FIG. 1 illustrates a typical situation for an n-type semiconductor where surface electric fields caused by the equilibration between electrolyte redox levels and the semiconductor Fermi level draws photoinduced holes to the semiconductor surface to oxidize redox species D- to D while electrons pass through the external circuit to reduce A+ to A at the counter electrode. At steady state, the electronic current I.sub.e in the external circuit is matched by an equal and opposite ionic current I.sub.i in the electrolyte. In summary, the semiconductor acts as a photosensitizer for carrying out the reaction D-+A+.fwdarw.D+A. For example, TiO.sub.2 sensitized photodecomposition of formic acid and other carboxylic acids in aqueous environments is well known.
Such PEC cells have a number of applications. Using the example of FIG. 1, if the reduced species is at a higher energy than the oxidized species, there is a net storage of incident radiant energy as chemical energy, as in a photoelectrosynthetic cell. The PEC splitting of H.sub.2 O is an example. If the recuced species is at a lower energy than the oxidized one, no energy has been stored. Rather the PEC system has catalyzed a thermodynamically downhill reaction. An example is the photocatalytic decomposition of acetic acid to methane and carbon dioxide.
In a regenerative cell, the species oxidized at the semiconductor is also reduced at the counter electrode so that there is no net change in the energy stored in the electrolyte. An example of this type of reaction occurs if O.sub.2 is dissolved in the electrolyte in which case O.sub.2 would be reduced to H.sub.2 O or OH.sup.- at the cathode while H.sub.2 O or OH.sup.- would be oxidized to O.sub.2 at the anode. However, the current in the external circuit can be used to drive an electrical load, as in a photovoltaic cell. These and other device possibilities have been described by Bard (A. J. Bard, Science 207, 139 (1980)).
One advantage of the PEC cell over the solid state photovoltaic cell is that it is produced quickly by simply immersing the semiconductor in the electrolyte. Secondly, light is absorbed in the region of the surface electric field of the semiconductor which causes the electron and hole to separate before they recombine. In solid state cells, the high electric field separation region is frequently at a greather distance from the surface of the semiconductor. This necessitates higher quality and higher cost material so as to allow the carriers to diffuse to this region before they recombine.
A microscopic version of a prior art PEC cell is shown schematically in FIG. 2. Here microscopic semiconductor powder grains 20 are dispersed in an electrolyte solution 21 again containing redox species D- and A+. Typically, these powder grains are submicron in dimension. The external circuit and counter electrode of FIG. 1 are replaced by a piece of catalytic material 22, typically a metal such as platinum, attached to a region of the semiconductor. The attachment can be accomplished by photochemical means. When the powder dispersion is illuminated, processes occur similar to those described for the macroscopic system of FIG. 1. Holes are drawn to the semiconductor surface where they oxidize D- while electrons move to the catalyst region where they reduce A+. The electronic flow in the grain amounts to an electronic current I.sub.e while the current loop is completed in the electrolyte by ionic current I.sub.i. Thus, the grains act as microscopic short-circuited PEC cells. The powder has the advantage of providing much more reactive surface area. A disadvantage is not having an external circuit essential for photovoltaic and other electrical device applications. Writings by Gratzel et al (J. Kiwi, K. Kalyanasundaram, and M. Gratzel, Structure and Bonding 49, Springer-Verlag, Berlin, p. 37, (1982)) and Bard (A. J. Bard, J. of Photochem. 10, 59 (1979)) discuss many of the details of the microscopic system and the numerous modifications that can be attempted to make the system more effective under different circumstances.
In principle, typical electrochemical (EC) cells including PEC cells can be adapted for use as a sensor of one of the chemical species involved in the cell reaction. One way to do this involves a modification to the cell structure so that the species to be sensed could be isolated in the vicinity of the electrode where it is taken up into the electrolyte in one of the EC reaction steps. For example, O.sub.2 can be sensed with an arrangement where it is spatially isolated next to a cathodically polarized electrode in a cell with an appropriate electrolyte. The amount of current passing through the cell depends on the oxygen concentration near the electrode and provides a measure of the oxygen concentration. Such a cell has been described by L. C. Clark Jr., R. Wold, D. Granger, and F. Taylor, J. Appl. Physiol. 6, 189 (1953). A similar arrangement could be made with the PEC cell shown in FIG. 1. In this case, the O.sub.2 would have to be isolated in a region near counter electrode 13. For the appropriate illumination intensity, counter electrode 13 would become cathodically polarized and the amount of current drawn in the external circuit would provide a measure of the oxygen concentration. Numerous chemical species could be sensed by this means. Using the macroscopic structure of FIG. 1, the PEC method has no major advantage over the EC method except where an optical source of power would be advantageous. The use of the microscopic PEC system for such a sensor is obviated by the absence of an ability to make a significant electrical contact with the dispersed semiconductor powder. The present invention describes a new structure and method for making a PEC gas sensor. The structure is especially suited to gas phase sensing and the PEC method in this case provides advantages over other EC methods.